Implement NTK scorer

Introduced in _Zhaoxuan Wu, Yao Shu, and Bryan Kian Hsiang Low, “[DAVINZ: Data Valuation Using Deep Neural Networks at Initialization](https://proceedings.mlr.press/v162/wu22j.html),” in Proceedings of the 39th International Conference on Machine Learning (International Conference on Machine Learning, PMLR, 2022), 24150–76._

It's a scorer using the Neural Tangent Kernel on untrained networks with a correction to estimate generalisation error. Implementation available in the supp. materials of the paper.

(Maybe relevant, JAX library for NTK: https://github.com/google/neural-tangents, and [paper](https://proceedings.mlr.press/v162/novak22a/novak22a.pdf) about fast finite-width NTK computation)